The present invention relates to an electric heater for a motor vehicle, using the heat generated by power semiconductors as the heat source.
Such a heater is described in German Patent No. 34 42 350. With this known heater, the power semiconductor controls the electric drive motor. The power semiconductor is connected to a cooling body through which a liquid coolant flows, so the heat generated is transferred to the liquid coolant by heat exchange. The liquid coolant circulates in a closed line system having a pump and the actual heater installation.
The efficiency of this known electric heater is not especially great, because the heat generated by the power semiconductor must be converted repeatedly. In addition, the heater installation has a complicated design, depends on the engine current present and thus cannot be regulated independently of the latter.
The object of the present invention is to create an electric heater, where the efficiency is greatly increased with a simple design and independent regulation of heating power is possible.
This object is achieved according to a first embodiment of the present invention by connecting several branch circuits, each with one power semiconductor operated in high power loss operation, in parallel for generation of heat, or according to a second embodiment by connecting several branch circuits, each with two series-connected power semiconductors operated in high power loss operation, in parallel for generation of heat.
In these embodiments, the current is converted directly into heat by the power semiconductors, which greatly increases efficiency. Another advantage of the new heater is that no additional control module is needed for the heater. Installation of the heater in the motor vehicle is also greatly simplified. In addition, the cabling complexity and manufacturing costs of the new electric heater are also reduced.
No separate fuse protection for the heater in the vehicle electrical system is necessary. When starting operation of the heater, the high starting current surge can be prevented by a regulated smooth current rise. The new heating module can be cascaded in any desired fashion to increase the heating power and can also be integrated easily into a fan regulator.
To protect the power semiconductors, one embodiment provides for a switching device that responds to overload to be connected in series with the power semiconductor in each branch circuit. In the event of a fault, the branch circuit affected can be shut down with this switching device without having to lose heater function as a whole. Heating power is reduced only by the ratio of defective branch circuits to total branch circuits.
According to another embodiment, regulation of the heating power is easily made possible by the fact that the power output by the power semiconductors can be regulated individually by a common predetermined setpoint and by actual values derived from the power semiconductors, or by the fact that the powers output by the respective first power semiconductors of the branch circuits can be regulated individually by a common predetermined setpoint and by actual values derived at these power semiconductors, and the powers output by the respective second power semiconductors can be regulated individually by a fixed predetermined control voltage and by actual values derived at these power semiconductors.
If the branch circuits are to supply power at the output to a low-resistance series resistor as a load impedance, then the heat generated by the series resistor can contribute to an increase in heating power. Each power semiconductor can supply power to an individual series resistor. All the power semiconductors may also supply power to a common series resistor, or groups of power semiconductors may each be connected to a group-individual series resistor.
The switching devices for interrupting the branch circuits can be implemented in various ways. Thus, according to one embodiment, the switching devices may be designed as a printed conductor part of the branch circuits which burn out in the event of a fault at the elevated current occurring in the respective branch circuit. The same effect can also be achieved by looping the switching devices as shunts into the branch circuits, which burn out in the event of a fault at the elevated current occurring in the respective branch circuit, in which case the shunt can also be used to derive another control signal. The branch circuit can also be interrupted by using the connecting wires of the power semiconductors which burn out in the event of a fault at the elevated current occurring in the respective branch circuit.
A controlled reduction or interruption in the current in a defective branch circuit occurs when measures are taken to ensure that in the event of a short circuit in one of the two power semiconductors connected in series in a branch circuit, an additional control signal can be derived from the defective branch circuit to reduce the power output by the respective second power semiconductor or switching it to a disconnect status. The control signal picked off at the shunt can be used as the control signal.
The structural design of the new electric heater can be simplified by designing it as a heater module, with the power semiconductors mounted in thermal contact on a cooling body, with the heat transfer via the cooling body being improved.
Simple temperature monitoring can be achieved with the electric heater by the fact that the power semiconductors and/or the cooling body are monitored by temperature sensors to detect whether a predetermined maximum temperature is exceeded, and by the fact that the output signals of the temperature sensor(s) reduce the power output by the respective power semiconductors or all the power semiconductors or switch them to a disconnect status. If the power semiconductors are monitored by individual temperature sensors, the expense of this is reduced by integrating the temperature sensors into the power semiconductors.